1. Field of the Invention
The present invention relates to fuel vapor processors, in particularly to fuel vapor processors mounted on a gas vehicle for preventing a large volume of fuel vapor from flowing into the atmosphere.
2. Description of the Related Art
A fuel vapor processor generally has a canister temporarily trapping fuel vapor vaporized in a fuel tank, a suction unit suctioning the fuel vapor from the canister, and a separator selectively passing the fuel vapor in a fuel vapor containing gas in order to concentrate the fuel vapor. The concentrated fuel vapor is returned into the fuel tank due to action of the suction unit.
Japanese Laid-Open Patent Publication No. H11-93784 discloses a fuel vapor processor where fuel vapor vaporized in a fuel tank is introduced into a liquefier and then the fuel vapor passing through the liquefier is introduced into and trapped in a canister. The fuel vapor temporarily trapped in the canister is suctioned due to suctioning power generated by a suctioning pump as a suction unit and is removed from the canister. Between the canister and the suctioning pump, a separation membrane as a separator is provided in order to concentrate the fuel vapor. The concentrated fuel gas (mainly containing fuel vapor) is flowed into the fuel tank via the liquefier. This fuel vapor processor is called as purge-less evaporation system where the fuel vapor is not purged into an engine and is returned into the fuel tank. The canister has an air communicating port communicated with the atmosphere such that a diluted fuel gas (mainly including air and not passing through the separation membrane) is introduced into the canister through the air communicating port together with ambient air.
The separation membrane cannot separate the fuel vapor from air completely. Separation performance of the separation membrane varies depending on a flow volume of the fuel vapor containing gas supplied to the separation membrane. As the flow volume of the gas supplied to the separation membrane increases, a concentration of the fuel vapor in the diluted gas also increases. In detail, as the gas volume supplied to the separation membrane increases, the volume of the fuel vapor required to be processed per unit of time increases. Therefore, when the very large amount of the fuel vapor is supplied to the separation membrane, it is difficult to pass the supplied fuel vapor through the separation membrane completely. Here, the volume of the gas supplied to the separation membrane is equal to a total volume of the diluted gas and the concentrated gas. Thus, when the flow volume of the concentrated gas increases, the density of the fuel vapor in the diluted gas decreases and the flow volume of the diluted gas decreases. Accordingly, when the flow volume of the fuel vapor separated from the fuel vapor containing gas increases, the volume of the diluted gas not passing through the separation membrane decreases.
The fuel vapor processor disclosed in Japanese Laid-Open Patent Publication No. H11-93784 does not have a regulator configured to control the amount of the fuel vapor containing gas supplied to the separation membrane (separator). In this case, the volume of the fuel vapor in the diluted gas varies depending on the volume of the fuel vapor containing gas supplied to the separation membrane. So, when the diluted gas contains the large volume of the fuel vapor and is introduced into the canister through the air communicating port, the large volume of the fuel vapor in the diluted gas mainly adsorbs onto adsorbent, which is contained in the canister and disposed near the air communicating port. Then, when the fuel vapor processor (suction pump) is stopped, some of the fuel vapor trapped near the air communicating port can flow into the atmosphere through the air communicating port.
In addition, in a condition that the fuel vapor processor is stopped, a slight amount of the diluted gas can be generated from the separation membrane, and such diluted gas is not introduced into the canister and is released into the atmosphere.
In the fuel vapor processor, the diluted gas can be introduced into the engine instead of into the canister. In such case, because the concentration of the fuel vapor in the diluted gas is not constant, fuel-air ratio in the engine is not fixed. Thus, there is a need for improved fuel vapor processors.